Catalyst and method for clarifying exhaust gas

ABSTRACT

A proton type β zeolite is used as catalyst. Nitrogen oxides in the exhaust gas containing excessive oxygen is reduced/removed by making the exhaust gas contact with the catalyst under the existence of methanol and/or dimethyl ether as reducing agent. It is desirable that a SiO 2 /Al 2 O 3  molar ratio of the proton type β zeolite is within 20-70. Thereby, the present catalyst has an excellent denitration performance and durability even against the exhaust gas containing sulfur oxides, and the denitration performance does not deteriorate even when the exhaust gas is at comparatively low temperature of 300-400° C.

TECHNICAL FIELD

The present invention relates to an catalyst for purifying exhaust gaseffective for removing nitrogen oxides (NOx) contained in various typesof combustion exhaust gas from boilers, diesel engine motors or dieselengine motor vehicles and from industrial facilities and a method forpurifying the exhaust gas using the catalyst.

BACKGROUND ART

Nitrogen oxides (NOx) such as nitrogen monoxide and nitrogen dioxide arecontained in various types of exhaust gas from factories, electric powerfacilities, other industrial facilities and motor vehicles. This NOx notonly affects especially on the respiratory system of human body but alsocauses acid rain seen as a problem in global environment conservation.Therefore, a technical development effective for removing nitrogenoxides in various types of exhaust gas is desired.

Among methods for removing such nitrogen oxides known so far, there area ternary catalytic method being used in exhaust gas treatment of amotor vehicle (a gasoline-powered vehicle) and a selective catalyticreduction method using ammonia as reducing agent. But the ternarycatalytic method cannot apply to the exhaust gas containing oxygen inexcess of theoretical amount necessary to completely oxidize an unburnedhydrocarbon or carbon monoxide remaining in the exhaust gas.

Meanwhile, among methods for reducing/removing NOx from the exhaust gascontaining excess oxygen known so far, there is a selective catalyticreduction method using a catalyst comprising V₂O₅—TiO₂ and using ammoniaas reducing agent. In this way, however, it is not easy to care forbecause of using highly smelling and harmful ammonia, and facilitiesgrow in size because it needs a special apparatus for limiting emissionsof unreacted ammonia. Therefore, it is not adequate to apply to asmall-sized exhaust gas source or mobile source, and undesirable ineconomical efficiency.

In recent years, it has been reported that using an unburned hydrocarbonas a reducing agent remaining in the lean-burned combustion exhaust gascontaining excess oxygen (ex. diesel engine) can accelerate a reductionreaction of NOx in the exhaust gas. Since this report, various catalyststo accelerate the reduction reaction of NOx have been developed, andmany reports have been made, for example, alumina and transitionmetal-impregnated almina are effective for a reduction/removal reactionof NOx which uses hydrocarbons as reducing agent.

As an example of a catalyst which reduces/removes nitrogen oxides in thecombustion exhaust gas containing excess oxygen with using suchhydrocarbons as reducing agent, besides alumina and transitionmetal-impregnated alumina, a reduction catalyst which comprises aluminaor silica-alumina including 0.1-4 weight % Cu, Fe, Cr, Zn, Ni or V of0.1-4 has been reported (Refer to JP KOKAI Hei 04-284848).

Furthermore, it has been reported that the reduction reaction of NOxproceeds even in a low-temperature range of 200-300° C. when aluminaimpregnated with Pt and the like. (Refer to JP KOKAI Hei 04-267946, JPKOKAI Hei 05-68855 and JP KOKAI Hei 05-103949). However, the catalystusing such precious metals has defect that it is difficult to getselectively proceed a reduction reaction into harmless N₂ because acombustion reaction of hydrocarbon which is a reducing agent is promotedexcessively or because N₂O which is seen as one of causative agent ofglobal warming is side-produced in large quantity.

Furthermore, it is reported that alumina and the like impregnated withsilver, with using hydrocarbon as reducing agent in the exhaust gascontaining excess oxygen, makes NOx reductive reaction proceedselectively (Refer to JP KOKAI Hei 04-281844). After this report, lotsof similar methods for NOx reduction/removal using the catalystcontaining silver have been developed and reported (Refer to JP KOKAIHei 04-354536).

However, each method for purifying the exhaust gas using NOx removalcatalyst has a problem that, in the exhaust gas containing sulfur oxidesand excess oxygen, the NOx removal performance deteriorates remarkablyand the practicable durability is insufficient. Furthermore, it has alsoa problem that NOx removal performance is low if the majority of theexhaust gas is at relatively low temperature of 300-400° C.

Furthermore, the method for the NOx reduction/removal under coexistenceof organic compounds which uses a hydrogenated zeolite catalyst orzeolite catalysts impregnated with V, Cr, Mn, Fe, Co, Ni and the likehas been reported and, as the zeolite, Y-type zeolite, L-type zeolite,offretite-erionite mixed crystal-type-zeolite, ferrierite-type zeoliteand ZSM-5-type zeolite are shown (Refer to JP Patent No. 2139645).Furthermore, the method for the NOx reduction/removal in the presence ofmethanol which uses a proton-type zeolite has also been reported and, asthe zeolite, Y-type zeolite, ZMS-5-type-zeolite and mordenite are shown(Refer to JP Patent No. 2506598).

However, each method for the NOx reduction/removal which usesabove-mentioned specific zeolite catalysts can not obtain thepracticably satisfying NOx removal performance, and at present, has notbeen put into practical use.

An object of this invention is, in consideration of such circumstances,to provide a catalyst for purifying exhaust gas, having an excellent NOxremoval performance and durability even against the exhaust gascontaining sulfur oxides, and having a high denitration performance andan excellent practicality even when the exhaust gas is at relatively lowtemperature of 300-400° C., and to provide a method of purifying exhaustgas using the catlyst.

DISCLOSURE OF THE INVENTION

To achieve the above object, the catalyst for purifying exhaust gasaccording to the present invention is able to reduce and remove nitrogenoxides in the exhaust gas in which excessive oxygen exsists, under theexistence of methanol and/or dimethyl ether, is characterized in that itconsists of a proton type β zeolite.

Furthermore, the method for purifying the exhaust gas according to thepresent invention, in order to reduce and remove nitrogen oxides in theexhaust gas, comprises a step of making the exhaust gas in whichexcessive oxygen exists contact with the proton type β zeolite catalystunder the existence of methanol and/or dimethyl ether which is areducing agent.

In the exhaust gas purifying catalyst and the method for purifying theexhaust gas according to the present invention, it is desirable that aSiO₂/Al₂O₃ molar ratio of the proton type β zeolite is within 20-70.

The present invention can provide a catalyst for purifying exhaust gashaving an excellent NOx removal performance and durability, and by usingthe catalyst, it is possible to effectively remove NOx in the exhaustgas in which excessive oxygen exists. Besides, the exhaust gas purifyingcatalyst according to the present invention shows a high NOx removalefficiency even against the exhaust gas containing a large amount ofsulfur oxides, and it is quite excellent in practice because thedenitration performance does not deteriorate even when the exhaust gasis at relatively low temperature of 300-400° C.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention, at the denitration of nitrogen oxides (NOx) inthe exhaust gas in which excessive oxygen exists, uses a proton type βzeolite as a catalyst. As the zeolite, besides type β, there are manytypes such as type Y, L, ZSM-5 (FMI), mordenite and the like, and amongthem, it is cleared that the proton type β zeolite which is ahydrogenated β zeolite is especially quite effective for purifying theexhaust gas containing sulfur oxides. That is, by using proton type βzeolite, the nitrogen oxides in the exhaust gas containing sulfur oxidescan be reduced and removed under the existence of methanol and/ordimethyl ether.

The proton type β zeolite is made, for example, by treating β zeolitewith aqueous ammonium nitrate solution and the like so as to be ammoniumtype, and thereafter burning it to make ammonia emitted. In addition,the proton type β zeolite can be molded and can be formed as variousshape catalysts in accordance with usage.

It is desirable that a silica (SiO₂)-alumina (Al₂O₃) ratio of the protontype β zeolite is, in terms of the denitration performance which removesNOx, within 20-70 in molar ratio. Above 40 of the molar ratio, thedenitration performance tends to deteriorate, and, considering stabilityagainst heat and vapor, it is more desirable that a SiO₂—Al₂O₃ molarratio is within 20-40.

The proton type β zeolite catalyst can be molded in various shapes suchas spherical, honeycomb, pellet state and the like by conventionalmolding methods. These shapes and sizes may be selected arbitrarilyaccording to use conditions of catalyst. Furthermore, a catalyst can bemade by means of wash-coating or otherwise proton type β zeolite on thesurface of a support plate having fireproof construction with somethrough-holes in the flow direction of the exhaust gas.

As the method for purifying the exhaust gas containing nitrogen oxidesin which excessive oxygen exists, the exhaust gas may be contacted withthe above-mentioned proton type β zeolite catalyst of the presentinvention under the existence of methanol and/or dimethyl ether. Theamount of methanol and/or dimethyl ether coexisting in the exhaust gasas reducing agent may be selected arbitrarily according to the NOxremoval efficiency or running cost which are operationally required, buttypically, it is desirable that the molar ratio (in carbon (C)) of theexhaust gas to nitrogen oxides is 0.5-5 degree.

As exhaust gases containing nitrogen oxides to which the presentinvention is applied, there are gases exhausted from various combustionfacilities such as boiler or the like, internal combustion engines ofdiesel engine motor vehicles or stationary diesel engines or the like,and industrial facilities such as nitric acid production facility. Theseexhaust gases generally contain reducing component such as CO, HC(Hydrocarbon) and H₂, and oxidizing component such as NOx and O₂, butcontain oxygen amount in excess of stoichiometric amount necessary forcomplete oxygen reduction reaction of the oxidizing component. NOx inthe exhaust gas in which excessive oxygen exists is reduced anddecomposited into N₂ and H₂O, by making the exhaust gas contact with thecatalyst of the present invention under the existence of methanol and/ordimethyl ether.

It is desirable that a gas space velocity (SV) in the method forpurifying the exhaust gas using the catalyst according to the presentinvention is, not to be considered limited to, 1,000-100,000 mh⁻¹.Furthermore, even when the exhaust gas is at relatively low temperatureof 300-400° C., an excellent denitration performance can be obtainedsubstantially equivalent to the case at high temperature. Besides, thecatalyst of the present invention has an excellent denitrationperformance even against the exhaust gas containing sulfur oxides and isalso excellent in its durability.

Meanwhile, when the exhaust gas is treated by the method according tothe present invention, depending on reaction condition, unburnedmethanol or dimethyl ether and poor combustion product are exhausted inthe gas. In such a case, the methanol or dimethyl ether and poorcombustion product can be removed by making the exhaust gas contact withan oxidation catalyst which is for example supported by precious metalssuch as Pt or Pd.

Embodiment

A proton type β zeolite was obtained by burning NH₄ type β zeolite(SiO₂/Al₂O₃ molar ratio: 27) on the market at 450° C. for 5 hours. Thiszeolite was pressure-formed, grained to the particle size of 350-500 μm,and defined as Catalyst 1 of the present invention. Furthermore, anotherproton type β zeolite was obtained by above-mentioned method except forusing NH₄ type β zeolite (SiO₂/Al₂O₃ molar ratio: 37) on the market.This zeolite was defined herein as Catalyst 2 of the present invention.

Correspondingly, as comparative example, the under-mentioned CatalystsC1-C4 were prepared. That is, Catalyst Cl is a proton type β zeolitewhich is obtained by burning a commercially available NH₄ type β zeolite(SiO₂/Al₂O₃ molar ratio: 75) at 450° C. for 5 hours. Catalyst C2 is aproton type mordenite which is obtained by burning a commerciallyavailable NH₄ type mordenite (SiO₂/Al₂O₃ molar ratio: 20) at 450° C. for5 hours. Catalyst C3 is a proton type ZSM-5 which is obtained by burningNH₄ type ZSM-5 (SiO₂/Al₂O₃ molar ratio: 27) on the market at 450° C. for5 hours. Furthermore, Catalyst C4 is made of β zeolite carrying Co whichis obtained by mixing 100 g of ion-exchanged water with 1.3 g of cobaltacetate tetrahydrate, dispersing 10 g of proton type β zeolite(SiO₂/Al₂O₃ molar ratio: 27) obtained by the above-mentioned method ofCatalyst 1 into the solution, agitating at 60° C. for 12 hours, andthen, after filtering, wet-cleaning and drying at 110° C., burning at500° C. for 3 hours in the atmosphere. In addition, the amount of Co inthe Catalyst C4 was 2.7 weight % in metal to the whole catalyst.

Catalyst 1, 2 and comparative examples of Catalyst C1-C4 obtained by theabove-mentioned methods were filled into each reaction tube of steelmeasuring 15 mm in inner diameter so as to form catalyst body, and thesewere fixed on the atmospheric fixed bed flow reactor. Within eachreaction tube, as exhaust gas model, a mixed gas consisting of NO: 1,000ppm, O₂: 10%, methanol: 1,000 ppm, H₂O: 10%, SO₂: 100 ppm, and remnant:N₂ was supplied under the condition that the spacial velocity is 30,000h⁻¹, and the NOx removal performance of each catalyst was evaluated. Atthat time, gas temperature was changed to at 300, 350 and 400° C.,respectively.

Furthermore, the denitration performance of each catalyst was evaluatedin the same way by changing gas composition of the model exhaust gas.That is, the denitration performance was evaluated by changing methanol:1,500 ppm which was added in the above-mentioned exhaust gas model asreducing agent to dimethyl ether: 750 ppm, and by using Catalyst 1, C2and C3. Furthermore, the denitration performance was evaluated bychanging the reducing agent to propylene: 500 ppm and 1,000 ppm andusing Catalyst 1.

Meanwhile, as for the gas composition analysis of the reaction tubeexit, the NOx concentration was measured by a chemiluminescent nitrogenoxide analyzer, and the N₂O concentration was measured by a gasmatograph having a thermal conductivity detector Porapak Q column. Inany catalyst, N₂O was hardly detected at the gas reaction tube exit. TheNOx removal efficiency as NOx removal performance of catalyst wascalculated according to the following formula;NOx removal efficiency (%)=[(NOx concentration at reaction tube entry−NOx concentration at reactiontube exit)/(Nox concentration at reaction tube entry)]×100

The NOx removal efficiencies of the above-mentioned Catalyst 1, 2 andC1-C4 are shown in the following table 1. As is clear from the result,it is understandable that, Catalysts 1 and 2 according to the presentinvention, compared to comparison example Catalysts C1-C4, haveremarkably excellent NOx removal performance in the exhaust gascontaining excessive oxygen in which a large amount of sulfur oxidesexists, even at relatively low temperature of 300-400° C. Furthermore,it is understandable that in Catalyst 1 and 2 according to theinvention, by using methanol and/or dimethyl ether as reducing agent, anexcellent denitration performance is appeared. Meanwhile, it isunderstandable that Catalyst Cl which is a proton type β zeolite butSiO₂/Al₂O₃ molar ratio is 75 has an extremely small NOx removalefficiency.

TABLE 1 Reducing NOx removal efficiency (%) Sample Catalyst agent 300.°C. 350.° C. 400.° C. 1  Catalyst 1 Methanol 64 88 95 2  Catalyst 2Methanol 59 81 90 3  Catalyst 1 Dimethyl 64 84 93 Ether 4* Catalyst C1Methanol 18 27 35 5* Catalyst C2 Methanol 22 46 56 6* Catalyst C3Methanol 17 32 51 7* Catalyst C4 Methanol 42 66 67 8* Catalyst C2Dimetyl 24 44 58 Ether 9* Catalyst C3 Dimetyl 15 35 52 Ether 10* Catalyst 1 Propylene 27 40 61 (500 ppm) 11*  Catalyst 1 Propylene 29 4567 (1,000 ppm) (NOTICE) samples * on this table are comparativeexamples.

Next, the durability evaluation of Catalyst, by using theabove-mentioned Catalyst 1 according to the present invention andcomparative examples Catalyst C₂-C₄, is described below. That is, asexhaust gas model, a mixed gas consisting of NO: 1,000 ppm, O₂: 10%,methanol: 1,500 ppm or dimethyl ether: 750 ppm, H₂O: 10%, SO₂: 1,000 ppmand remnant: N₂ was supplied into each reaction tube which is arrangedin a similar way as described above under the condition that the gastemperature is 350° C. and the space velocity is 30,000 h⁻¹ for 20hours.

After the above-mentioned durability test, the exhaust gas model whichis similar in composition to the above model except for containing 100ppm of SO₂ was supplied into each reaction tube under the condition thatgas temperature is 350° C. and the space velocity is 30,000 h⁻¹ and thenthe NOx removal efficiency was obtained in a similar way as describedabove. The result obtained is shown in the following table 2.

TABLE 2 NOx removal efficiency (%) Before After Sample Catalyst Reducingagent durability test durability test 12  Catalyst 1 Methanol 88 83 13 Catalyst 1 Dimethyl Ether 84 80 14* Catalyst C2 Methanol 46 41 15*Catalyst C3 Methanol 32 30 16* Catalyst C4 Methanol 66 54 17* CatalystC3 Dimethyl Ether 35 32 (NOTICE) samples * on this table are comparativeexamples.

As is clear from the result, even in the 20-hour endurance test by theexhaust gas containing SO₂ of high concentration, the catalyst which ismade of the proton type β zeolite catalyst according to the presentinvention keeps a high activity and has an excellent durability.Meanwhile, it is understandable that, as for the catalysts ofcomparative examples, proton type mordenite catalyst C2 and proton typeZSM-5 catalyst C3 having low denitration performance show favorabledurability, but Catalyst C4 having comparatively excellent denitrationperformance, which is made of β zeolite which supports Co is inferior indurability.

1. A catalyst for purifying exhaust gas, which reduces nitrogen oxidesin an exhaust gas containing excessive oxygen under the existence ofmethanol and/or dimethyl ether, wherein the catalyst consists of aproton type β zeolite having a SiO₂/Al₂O₃ molar ratio within 20-70wherein the catalyst has substantial denitrification performance anddurability.
 2. A method of purifying exhaust gas, wherein said methodincludes reducing nitrogen oxides in the exhaust gas containingexcessive oxygen therein, comprising contacting the exhaust gas with acatalyst consisting of a proton type β zeolite catalyst in the presenceof methanol and/or dimethyl ether as reducing agent, wherein the protontype β zeolite has a SiO₂/Al₂O₃ molar ratio within 20-70 wherein thecatalyst has substantial denitrification performance and durability. 3.A method of purifying exhaust gas, wherein said method reduces nitrogenoxides in the exhaust gas containing excessive oxygen therein,comprising contacting the exhaust gas with a catalyst in the presence ofmethanol and/or dimethyl ether as reducing agent, wherein the catalystcomprises a proton type β zeolite catalyst having a SiO₂/Al₂O₃ molarratio within 20-70 wherein the catalyst has substantial denitrificationperformance and durability.